1. Field of the Invention
The present invention relates to a technique of reducing fixed pattern noise in a captured image.
2. Description of the Related Art
An image sensor such as a CCD sensor or CMOS sensor sometimes generates streak-like fixed pattern noise called a “vertical streak” in a captured image owing to nonuniformity of a dark current in a vertical transfer root and degradation in transferring. As a method for correcting the generated fixed pattern noise, a technique, where the output signal (black image) of an image sensor that is obtained in a light-shielded state is stored in advance, and subtracted from a captured image (black subtraction process), is known.
However, the black image contains not only fixed pattern noise but also random noise. In an image obtained by subtracting a black image from a captured image, random noise stands out much more than in an image before subtraction. To prevent this, a technique is known whereby random noise from a black image is reduced to generate an extraction image which extracts streak-like fixed pattern noise.
Japanese Patent Laid-Open No. 2007-028026 discloses a technique of reducing random noise by applying a median filter using a row or column of an image sensor as one process unit. Japanese Patent Laid-Open No. 2009-033321 discloses a technique of reducing random noise in a region used for image capturing. More specifically, a light-shielded region not used for image capturing is set at the periphery of an image sensor. A dark current amount in the region used for image capturing is calculated from a dark current amount in image capturing that has been measured in the light-shielded region.
However, delay lines with ten-odd or more taps are sometimes necessary to sufficiently reduce random noise by the method of applying a filter for reducing random noise for each row or column, like Japanese Patent Laid-Open No. 2007-028026. This may increase the circuit scale.
The method of setting a light-shielded region around an image sensor, like Japanese Patent Laid-Open No. 2009-033321, may decrease the number of effective pixels of the image sensor or increase the circuit scale owing to expansion of the image sensor for setting the light-shielded region. In Japanese Patent Laid-Open No. 2009-033321, a dark current amount in the region used for image capturing is linearly estimated and calculated. Depending on the trend of change of a dark current amount in the region used for image capturing, no random noise may be appropriately reduced.